1. Field of the Invention
The present invention relates to a magnetic sensor including giant magnetoresistive elements and a method for manufacturing the same.
2. Description of the Related Art
A generally known giant magnetoresistive element comprises a spin-valve film including a fixed magnetization layer, a free layer whose magnetization direction is changed in response to an external magnetic field, and a nonmagnetic conductive spacer layer. The fixed magnetization layer includes a pinned layer and a pinning layer for fixing the magnetization direction of the pinned layer, and the spacer layer is disposed between the pinned layer and the free layer. Since the pinned layer of the fixed magnetization layer comprises a single ferromagnetic layer (for example, a CoFe layer), the fixed magnetization layer is hereinafter referred to as the “single-layer-pinned fixed magnetization layer” and the spin-valve films including the single-layer-pinned fixed magnetization layer is hereinafter referred to as the “single-layer-pinned spin-valve film”, for the sake of convenience. A giant magnetoresistive element including the single-layer-pinned fixed magnetization layer is hereinafter referred to as a “conventional GMR element”.
The resistance of the conventional GMR element varies depending on the angle formed by the magnetization directions of the pinned layer and the free layer. Specifically, the resistance of the element varies in response to the component of an external magnetic field along the magnetization direction of the pinned layer. Therefore, the element detects magnetic fields in the direction along the fixed magnetization direction of the pinned layer (more properly, the direction antiparallel to the magnetization direction of the pinned layer). In order to fix the magnetization direction of the pinned layer, magnetic field heat treatment is performed in which, for example, a composite film including a ferromagnetic layer intended to act as the pinned layer and an antiferromagnetic layer intended to act as the pinning layer is heat-treated at a high temperature while a magnetic field oriented in a predetermined direction is applied to the composite film.
As shown in FIG. 45A, a magnetic sensor using the conventional GMR element generally includes two conventional GMR elements 101 and 102 detecting a magnetic field in a predetermined direction and another two conventional GMR elements 103 and 104 detecting a magnetic field in the direction antiparallel to the predetermine direction. These GMR elements are connected in a full-bridge configuration so as to output the potential difference V between the points shown in the figure. FIG. 45B shows the output V of the magnetic sensor shown in FIG. 45A in response to an external magnetic field H in its magnetic-field-detecting direction.
This bridge configuration allows the known magnetic sensor to produce high output even for a small magnetic field. In the known magnetic sensor, the temperatures of the GMR elements vary evenly, and the resistances of the GMR elements vary evenly, accordingly. For example, if the temperature of a GMR element increases, the temperatures of the other GMR elements increase evenly and thus the resistances of all the GMR elements varies evenly. Thus, the output V is not easily affected by the changes in temperature of the GMR elements, and the magnetic sensor can accurately detect external magnetic fields even if the temperatures of the GMR elements are varied (as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2004-163419).
The magnetization direction of the pinned layer, which determines the magnetic-field-detecting direction, is the same as the direction of a magnetic field applied to the layers which will become the fixed magnetization layer during the magnetic field heat treatment. In order to form a plurality of conventional GMR elements detecting magnetic fields in antiparallel directions for use in the bridge configuration, antiparallel magnetic fields must be applied to a substrate having a plurality of films which will become the conventional GMR elements. Furthermore, for a magnetic sensor capable of detecting the components of a magnetic field along two perpendicular directions (for example, X-axis and Y-axis directions), conventional GMR elements detecting components of a magnetic field in the positive X-axis direction, positive Y-axis direction, negative X-axis direction, and negative Y-axis direction are provided on a very small substrate. Thus, magnetic fields oriented in these four directions must be applied to the substrate having films which will become the conventional GMR elements during the magnetic field heat treatment. However, it is difficult to generate such magnetic fields oriented in different directions from one another in a small area.
The above-cited Japanese Unexamined Patent Application Publication No. 2004-163419 has disclosed a method for manufacturing a magnetic sensor, using the following sensor structure and magnet array. Specifically, films which will become four pairs (eight in total) of conventional GMR elements 101 to 108 are formed in the vicinities of the four edges of a substantially square substrate 100a, as shown in the plan view in FIG. 46.
The magnet array includes rectangular solid permanent magnets arrayed in a tetragonal lattice manner. The permanent magnets are arrayed in such a manner that their end surfaces on one side are present in substantially the same plane and the end surfaces of any two adjacent permanent magnets have magnetic polarities opposite to each other. FIG. 47 is a perspective view of some of permanent magnets 110 in the magnet array. FIG. 47 shows that the upper side of the magnet array and magnetic fields generated by the magnets in four directions from an N pole to S poles.
For performing the magnetic field heat treatment, the substrate 100a having the films which will become the conventional GMR elements is disposed over the upper side of the magnet array. The magnetic fields in the four directions generated from the upper side of the magnet array are applied, for heat treatment, to the films which will become the conventional GMR elements, as shown in FIG. 48. A magnetic sensor 100 shown in FIG. 46 is thus produced.
The conventional GMR elements 101 to 104 of the magnetic sensor 100 detect the component of a magnetic field along the X-axis direction. The magnetization directions of the pinned layers of the conventional GMR elements 101 and 102 are fixed in the negative X-axis direction. The magnetization directions of the pinned layers of the conventional GMR elements 103 and 104 are fixed in the positive X-axis direction. In general, the conventional GMR elements 101 to 104 are connected in a full-bridge configuration, as shown in FIG. 45, to form an X-axis magnetic sensor for detecting magnetic fields in the X-axis directions.
The conventional GMR elements 105 to 108 detect the component of a magnetic field along the Y-axis direction. The magnetization directions of the pinned layers of the conventional GMR elements 105 and 106 are fixed in the positive Y-axis direction. The magnetization directions of the pinned layers of the conventional GMR elements 107 and 108 are fixed in the negative Y-axis direction. The conventional GMR elements 105 to 108 are connected in the same full-bridge configuration as the conventional GMR elements 101 to 104, and thus form a Y-axis magnetic sensor for detecting magnetic fields in the Y-axis directions.
In such perpendicular bidirectional (detecting) magnetic sensor, the conventional GMR elements are disposed in the vicinities of the four edges of the substrate 100a, and accordingly it is difficult to miniaturize the magnetic sensor (chip) sufficiently.
In a magnetic sensor in which the conventional GMR elements are arranged with long distances, the conventional GMR elements are unevenly deformed by stresses unevenly put thereon if the substrate 100a or a resin coating or the like covering the substrate 100a is deformed by heat, external stresses, and so forth. Consequently, the resistances of the conventional GMR elements connected in a bridge configuration are individually varied, and thus the bridge circuit of the magnetic sensor becomes imbalance. As a result, the magnetic sensor 100 cannot accurately detect magnetic fields.
Furthermore, since the distance between the conventional GMR elements in the magnetic sensor is long, the lengths of wires forming the full-bridge configuration are increased, and accordingly losses due to the resistance of the wires are increased.